Abstract: An analysis was conducted on an incident where the secondary system voltage of a 500 kV capacitive voltage transformer (CVT) was zero after being put into operation. Through actual measurements of the secondary coil voltage, the fault was determined to be a fault in the CVT itself. After the power outage, a dielectric loss and capacitance test was conducted on the CVT. It was determined that the direct cause of the incident was that the selection switch position was marked reversely when the CVT was shipped, and the selection switch was mistakenly set to the "test" position before being put into operation. The primary terminal of the intermediate transformer was grounded, causing the secondary coil voltage to return to zero. Finally, the method for determining the position of the selection switch is summarized, and corresponding preventive measures are proposed.